In recent years, portable electronic devices such as laptop PCs, mobile phones, and PDAs are in widespread use. Accordingly, there is a demand for a secondary battery used as a power source of these devices to be small-sized and to have high energy density, in order to allow these devices to be lighter and to become usable for longer hours. Conventionally, nickel-cadmium batteries, nickel-hydrogen batteries, or the like are used as major secondary batteries. However, due to the aforementioned demand for secondary batteries to be small-sized and to have high energy density, the use of lithium secondary batteries is increasing.
Currently, general lithium secondary batteries use lithium cobalt oxide as a positive electrode, a carbon electrode as a negative electrode, and a nonaqueous electrolyte solution as an electrolyte, which is obtained by dissolving lithium ions in an organic solvent such as propylene carbonate.
There is a well known general method for producing lithium cobalt oxide (LiCoO2) which is used as a positive-electrode active material for general lithium secondary batteries. In this method, lithium carbonate (Li2CO3) and cobalt hydroxide (Co(OH)2) are used as starting materials which are burned at a high temperature. However, in this method, the temperature needs to be as high as 700 to 900° C., resulting in a high production cost. Moreover, the recoverable reserves of cobalt are 8.4 million tons, which is an extremely small amount. Therefore, given a possible increase in the cobalt price in the future, a material alternative to LiCoO2 is sought.
In view of the above, lithium manganese oxides are drawing attention as alternatives to LiCoO2 since they are available at a relatively lower price and also favorable from the environmental point of view. Research and development have been actively conducted on lithium manganese oxides, such as spinel-type lithium manganese oxide (LiMn2O4) and tetragonal lithium manganese oxide (LiMnO2), as next-generation, low-cost, positive electrode materials.
However, in a method for producing such a lithium manganese oxide, a heating temperature and a cooling speed in slow cooling are important factors. In general, a method for producing a lithium manganese oxide requires a burning process which is performed at a high temperature of 700 to 800° C. (see Non-Patent Literature 1).
Accordingly, a sintered body obtained in this method tends to be one in which sintering having occurred among particles is uneven, and also, a resultant particle diameter is large, which is unfavorable. Moreover, the reactivity of a mixture of a manganese compound and a lithium compound is poor even if a burning process is performed thereon at a high temperature. Accordingly, a resultant composition tends to have lithium deficiency or oxygen deficiency. In order to avoid these problems, it is necessary to repeatedly perform the burning process and mechanical crushing, which results in an increase in the production cost. Since the burning process performed at a high temperature contributes to the increase in the production cost, research and development have been conducted to develop a method for producing a material at a lower temperature, aiming at lowering the cost. For example, one of the existing low-temperature synthesis methods is a hydrothermal synthesis method (see Non-Patent Literature 2 and Patent Literatures 1 and 2).
It is expected that the use of a hydrothermal synthesis method makes it possible to mass-manufacture positive-electrode active materials at a low temperature and at low cost.
Among such hydrothermal synthesis methods, Patent Literature 2 discloses a method for producing a lithium manganese oxide, the method including the steps of: causing a reaction between a manganese compound and an alkali to obtain a manganese hydroxide; causing the manganese hydroxide to be oxidized in an aqueous solvent or in a gas phase to obtain a manganese oxide; causing a reaction between the manganese oxide and a lithium compound in an aqueous solvent to obtain a lithium manganese oxide precursor; and heating and burning the precursor.
In the lithium manganese oxide obtained in this manner, each particle has a cubic shape and has a gap therein. Patent Literature 2 found out that if a lithium secondary battery is formed by incorporating therein the lithium manganese oxide as a positive-electrode active material, then the initial charge/discharge capacity of the battery is high, and also, the charge/discharge cycle characteristics of the battery are excellent.
Lithium-doped transition metal oxides obtained by methods disclosed in Non-Patent Literatures 1, 2 and Patent Literatures 1, 2 are each obtained in the form of powder. Therefore, in order to fabricate a positive electrode, it is necessary to mix the powder with a conductive agent and a binder to form a positive electrode slurry, and to apply the slurry onto a current collector such as an aluminum foil by means of a doctor blade or the like, thereby obtaining a current collecting ability.
However, in the fields of electric automobiles, trains, airplanes, etc., which involve heavy-duty industrial drive source applications, it is necessary to realize charging/discharging with very high current density. Therefore, it is preferred that a positive electrode has a wide surface area and that a composite material layer on the positive electrode is as thin as possible.
For example, one of such batteries, in which the surface area of an electrode is wide and an active material layer on the electrode is thin, is a battery using a fiber electrode (see Patent Literature 3).
In such a battery, the surface area of the electrode is significantly large. This realizes charging/discharging with high current density. However, it is difficult to evenly form a thin composite material layer on a positive electrode with a conventional coating method.
Moreover, if a lithium-doped transition metal oxide that is in the form of powder is used and a conventional coating method is used, then a resultant electrode is a flat, plate-shaped electrode. Such an electrode has a structure in which the surface area of the electrode is insufficient, and which does not allow an electrolyte solution to easily permeate, and which cannot mitigate a stress that occurs due to expansion/contraction in the volume of the active material.
If a carbon electrode is used as a negative electrode of a lithium secondary battery, then lithium ions are captured between carbon layers at the time of charging. Accordingly, apparent changes in the volume of the electrode in charge-discharge reactions are small. However, in the case of a carbon electrode, the available current density is low, and the theoretical value of capacity density is 372 mAh/g, which is also low. Moreover, in the case of a carbon electrode, the fabrication process is complex and the fabrication yield is low. Thus, there is a drawback of a high fabrication cost.
In terms of capacity density, metal lithium is the greatest. The theoretical value of the capacity density of metal lithium is 3860 mAh/g, which is high. The charge/discharge capacity of metal lithium is 10 times or more greater than that of a carbon electrode. However, if metal lithium is used as a negative electrode of a lithium secondary battery, a lithium dendrite is formed due to repeating charge-discharge reactions. This causes short-circuiting between electrodes and destruction of a separator, resulting in a sudden decrease in the charge/discharge cycle efficiency of the lithium secondary battery, and also resulting in a decrease in the safety of the battery.
In view of the above, Sn is drawing attention as a next-generation negative electrode material since Sn is inexpensive, has a relatively low effect on the environment, and has a theoretical capacity which is twice or more greater than that of a conventional carbon material (energy density of 994 mAh/g). However, the volume of Sn increases up to 3 to 4 times greater than its initial volume due to lithium occlusion and release which occur at the time of charging and discharging. Accordingly, the capacity is reduced to approximately 100 mAh/g after approximately 20 cycles. Thus, in the case of using Sn, the cycle life characteristics are very poor. Moreover, since Sn acts as a catalyst, there is a problem in that an electrolyte solution is decomposed.
In view of the above, attempts have been made to form a thin film of Sn or an alloy containing Sn, which is alloyed with Li, on a current collector formed of a material that is not alloyed with Li, and to use the thin film as a negative electrode material, thereby solving the above-described problems (see Patent Literature 4 and Patent Literature 5, for example). Patent Literature 4 discloses forming a Sn thin film on a copper plate current collector by an electrolytic plating method. Patent Literature 5 discloses forming a thin film, which is made of Sn, Zn, Sb, or an alloy containing these, on a copper foil by an electrolytic plating method.
Non-Patent Literature 3 discloses that by performing heat treatment on a Sn thin film previously formed by an electrolytic plating method on a Cu foil, it is possible to obtain a thin film having a gradient structure in which Cu atoms and Sn atoms interdiffuse at the Cu—Sn interface. To be specific, when a Sn thin film formed by plating a Cu foil with Sn is subjected to heat treatment at a temperature near the melting point of Sn, atomic interdiffusion occurs at the Cu—Sn interface. Eventually, a Cu—Sn alloy of Cu/Cu3Sn/Cu6Sn5/Sn or having a crystal structure of a similar composition is formed. The Cu6Sn5 alloy formed here is capable of reversible Li occlusion and desorption. Moreover, the volume of Cu6Sn5 changes less than the volume of Sn, and also, Cu6Sn5 does not act as a catalyst. Therefore, Cu6Sn5 is expected as a negative electrode material that solves the above-described problems unique to a Sn thin film.
In order to obtain a more favorable cycle life, Patent Literatures 6 and 7 disclose inventions in relation to a negative electrode. In these inventions, a Sn (or Sn alloy) plated coating is formed on a copper foil current collector, and then heat treatment is performed thereon. As a result, a copper-Sn intermetallic compound is formed as an intermediate layer between the copper current collector and the Sn (or Sn alloy) plated coating. However, if a charge/discharge test is performed with a cutoff voltage of 0-1V (Li potential), a capacity reduction to approximately 300 mAh/g is observed after approximately 50 cycles and the capacity is not constant. If the heat treatment is performed at a temperature higher than 190° C., delamination occurs at the interface between a copper plate current collector and a layer of which Cu3Sn is a main component. This results in a reduced cycle life.